README.byteorder

                 -- Byte order handling in libmseed --

The SEED 2.4 standard allows data only SEED (Mini-SEED) to be either
in big (most significant byte first) or little (least significant byte
first) endian byte order.  One exception is that Steim-1 and Steim-2
data compression are only defined as big-endian, that said libmseed
supports little-endian Steim compression schemes anyway.  While
libmseed supports all four combinations of big and little endian header
and data the surest way to avoid compatibility problems is to always 
create big endian Mini-SEED records (header and data).


Reading MiniSEED - how libmseed determines the byte order of a record:

The byte order of a record header including blockettes is determined
by checking if the record start year and day is a sane value (e.g. year
between 1900 and 2100 and day between 1 and 366).  The byte order of
encoded data samples is determined by the byte order flag in the
Blockette 1000, if a Blockette 1000 is not present the byte order is
assumed to be the same as the header.  To force the byte order
determination of either the header or data section of a record the
following environment variables can be set:

UNPACK_HEADER_BYTEORDER
UNPACK_DATA_BYTEORDER

These variables should be set to either 0 (little endian) or 1 (big
endian).  A programmatic equivalent of setting these environment
variables is provided via the following macros:

MS_UNPACKHEADERBYTEORDER(X)
MS_UNPACKDATABYTEORDER(X)


Writing MiniSEED - in what byte order libmseed creates records:

Normally the byte order of MiniSEED created by libmseed is controlled
via a flag in the API.  This byte order flag determines the ordering
for both the header and data sections of a record.  To force the byte
order of either the header or data section of a record the following
environment variables can be set:

PACK_HEADER_BYTEORDER
PACK_DATA_BYTEORDER

These variables should be set to either 0 (little endian) or 1 (big
endian).  A programmatic equivalent of setting these environment
variables is provided via the following macros:

MS_PACKHEADERBYTEORDER(X)
MS_PACKDATABYTEORDER(X)


Note that some interpretations of the SEED 2.4 format imply that
so-called little endian MiniSEED means that the record header is
little endian but that the data section is big endian (as the only
defined data encodings must be based on the SEED DDL which, in turn,
must be defined in terms of big endian).  Libmseed will not create
MiniSEED of this flavor by default but can be configured to do so by
setting the environment variables described above appropriately.